1. Field of the Invention
The present invention relates generally to electronic security systems of the type used to detect the removal of an article, such as a bundle of currency, from a protected premises and, more particularly, to such a security system wherein radio signals are transmitted near the exits from the premises for actuating a receiver concealed within the article as it is removed from the protected premises.
2. Description of the Prior Art
Electronic security systems incorporating packets disguised as bundles of paper currency have long been used by banks and other financial institutions to help deter robberies and to assist in the apprehension of hold-up assailants and in the recovery of stolen money. One such electronic security system is described in U.S. Pat. No. 3,781,860, issued Dec. 25, 1973, entitled "METHOD AND APPARATUS FOR INHIBITING ARTICLE THEFT", and assigned to the assignee of the present invention. The security system described in the aforementioned patent includes a transmitter for transmitting first and second radio signals of different frequencies in the vicinity of each exit from the bank. A radio receiver is concealed within a hollowed-out packet resembling a strapped bundle of bills. Also concealed within the packet is a battery to power the receiver and one or more explosive squibs. Upon receiving the first and second signals, the receiver causes the battery voltage to be applied to these squibs, resulting in an explosion for disbursing tear gas, dust, smoke, and tracer dye from canisters also concealed within the packet.
The packets are normally maintained in teller drawers within the bank and are handed out to a robber by a teller during a robbery along with real bundles of money. As the robber leaves the bank, the concealed receiver is brought within the field of the transmitted first and second radio signals and is actuated thereby. Other patents disclosing related security systems are U.S. Pat. Nos. Re. 27,618 and 3,828,341, the latter of which discloses circuitry for preventing the squibs from being fired until the receiver packet has been removed from the vicinity of the bank exit to lessen the likelihood of the packet exploding within the bank premises.
One of the goals of prior art security devices of the type described above has been to minimize current drain from the battery within the receiver packet to extend required servicing intervals and to enhance the reliability of the security system. Attempts to minimize power drain from the battery have taken various forms, including the use of a switch normally biased in an open condition by a pin mounted in the teller cash drawer and extending into the currency packet through a small hole in the lower portion thereof, as taught in U.S. Pat. No. Re. 27,618. It has also been known in the art to incorporate a magnetic reed switch within the currency packet and to install a magnetic keeper plate within the teller cash drawer for normally biasing the magnetic reed switch in an open condition so long as the currency packet is disposed atop the keeper plate as disclosed within U.S. Pat. No. 3,828,341. Such prior art techniques suffer from the disadvantage of depending upon mechanical elements more subject to failure than electronic components.
The aforementioned U.S. Pat. No. 3,781,860, discloses a purely electronic power switch adapted to minimize the drain of power from the battery. Within the receiver circuit disclosed therein, a first receiver is continuously powered by the battery for detecting a first radio signal. A normally open power switch is interposed between the battery and the remainder of the components within the receiver circuit. Only after the first signal is received is the state of the power switch altered to electrically couple the battery to the remainder of the electronic components within the receiver circuit.
In addition to minimizing power drain on the battery within the receiver packet, another objective when designing currency packets for such radio controlled security systems is to minimize the possibility of the currency packet being falsely triggered by stray repetitive signals radiated from electro-mechanical and computer equipment often found within banks and other financial institutions. On approach toward minimizing the likelihood of such false triggering is disclosed within the aforementioned patent, U.S. Pat. No. 3,781,860, wherein the receiver packet includes circuitry for receiving first, second and third frequency signals; the currency packet can be fired only if the first and second frequency signals are detected in the absence of the third frequency signal. Receipt of the third frequency signal is indicative of the presence of randomly generated noise, and the firing circuit is then disabled to guard against false triggering. However, it has been found that, in rare instances, relatively narrow band width signals of the aforementioned first and second frequencies are radiated by equipment within the bank without also generating the third frequency disabling signal. In such rare instances, the currency packets have been known to be triggered even though not brought within the field of the signals intentionally transmitted near the bank exit. At least one manufacturer known to the present inventor is currently marketing such a security system utilizing digital coding techniques to further minimize the possibility of false triggering.
Of course, when designing such a security system, the size and number of components within the receiver packet must be minimized to be relatively compact and light-weight for being easily concealed within what appears and feels to be an ordinary packet of strapped bills. The receiver circuitry must consume little power to avoid excessive drain on the battery and yet be sufficiently sensitive to minimize "body shadowing" effects, i.e., reductions in the transmitted signal levels when the assailant's body is interposed between the transmitter and the receiver.
In such prior art security systems, it has been found desirable to incorporate filters within the receiver circuits to minimize the band width of signals detected by each such receiver in order to reject stray signals. Consequently, it has also been found necessary to carefully tune the transmitter frequencies at the factory to insure that the transmitted radio signals fall within the band widths detected by the corresponding receiver circuits. It has also been found necessary to periodically align the transmitter frequencies with the band width filters incorporated within the respective receiver circuits at periodic service intervals in view of frequency drift caused by aging of the frequency determining components.
Accordingly, it is an object of the present invention to provide such a radio-controlled security system which minimizes the drain of power from the battery while remaining highly sensitive to radio signals transmitted in the vicinity of the exits from the bank.
It is another object of the present invention to provide such a security system which minimizes the likelihood of the currency packet being falsely triggered by stray signals generated by electro-mechanical and/or computer equipment found in banks and other financial institutions.
It is still another object of the present invention to provide such a security system wherein frequency alignment of the transmitter at time of manufacture and at periodic service intervals is virtually eliminated.
It is a further object of the present invention to provide such a security system wherein the currency packet receiver circuitry is highly compact, lightweight, and relatively inexpensive.
It is a further object of the present invention to provide such a security system which minimizes the power requirements and physical size of the associated transmitter.
It is a still further object of the present invention to provide such a security system which utilizes a digitally coded signal and wherein the digitally coded signal may be easily and conveniently varied for a given user or as between several different users.
These and other objects of the present invention will become apparent to those skilled in the art as the description thereof proceeds.